It is very common for battery packs which are used in portable communication devices, such as two-way radios, to have a thermistor and a battery capacity resistor. The thermistor is used by a battery charger during the charging of the battery, to determine if the battery is being charged properly. While the capacity resistor is used by the charger to determine the capacity of the battery, prior to the battery being charged. The battery charger upon determining the battery capacity (e.g., 1000 milli-amp-hour maH, etc.) will select the proper charging rate to use, in order to optimally charge the battery pack.
Referring to FIG. 1, there is shown a prior art battery charging scheme consisting of a charger 102, radio battery pack 106 and radio 104. Radio 104 contains positive (B+) and negative (B-) battery terminals which are coupled to radio battery 106 via battery contacts 116 and 114, respectively. Battery 106 contains one or more battery cells 108, which determine the voltage and current capacity of battery 106. Also included as part of the battery 106, are protection diode 118, a battery temperature indicator such as thermistor (Rt) 112 and a battery capacity indicator, such as resistor (Rc) 110.
Charger 102 consists of a charger monitor circuit 128, which can consist of a well known microprocessor or microcontroller as known in the art and appropriate control software. Charger monitor circuit 128 controls charger control circuit 130 which provides current to battery 106 in order to charge the battery. A control signal is transmitted by charger monitor circuit 128 to charger control circuit 130 via bus 140, the control signal informs charger control circuit 130 on how much current to source via line 129 to battery 106.
Charger monitor circuit 128 contains three analog to digital (A/D) ports 120, 122 and 124. A/D port 120 monitors the voltage on the B+ line. A/D port 122 senses the resistance of capacity resistor Rc 110 and A/D port 124 in turn senses the resistance of thermistor Rt 112, as its resistance changes once the battery begins charging. A/D ports 122 and 124 include external pull-up resistors which are used to determine the resistance of Rc 110 and Rt 112, by determining the voltage level at A/D ports 122 and 124, respectively.
The problem with the prior art charger system shown above is that typically, if the radio is turned "on", the radio standby current requirements are greater than the trickle charge current that a charger supplies to the battery. This problem is worsened in newer battery technologies such as Nickel Metal Hydride which are more sensitive to overcharge and therefore typically require lower trickle charge rates. Thus, a battery being charged with an attached radio that is left in the "on" condition, begins losing charge. The problem worsens if the user places the radio in a transmit mode or if a message is received by the radio causing the radio to enter into it's receive mode. In both the receive and transmit mode, the battery being charged loses even a greater amount of charge.
Most battery chargers for portable radios typically only include a limited number of lines which connect the charger to the battery under charge. For example, charger 102 and battery 106 have 4 lines connecting the charger 102 and battery 106. These lines being B+ line 132 which provides the charging current to the battery pack, Rc line 134 which is used to sense the capacity resistor 110, thermistor sense line 136 is used to sense battery temperature via changes in the resistance value of thermistor 112, and B- (ground) line 138. In most prior art battery charger systems there is no way for the charger to determine that the radio has changed state (e.g., the radio has been turned on, radio is receiving information, etc.) while the radio battery is being charged. Thus a battery being charged with an attached radio that is left on can actually discharge instead of charge. This is compounded further if the radio in the charger goes into the receive or transmit mode. Hence a means to compensate the battery for the additional radio state dependent current drain is necessary to assure that the battery is charged and maintained properly.
Presently, only vehicular adapters have this current compensation capability. This is because vehicular radio adapters interconnect to the radio's universal connector which allows for the monitoring of the radio data bus to determine the state of the radio. Once the state of the radio is determined, the proper compensation current can be supplied to the battery to offset for the radio's additional current drain. Desk top chargers until now have not had this capability due to the added expense required in interconnecting to the radio's universal connector. A need thus exists in the art for providing a simple and cost effective way of providing for radio state information to be sent to the charger while the radio battery is being charged. The apparatus provided also preferably should maintain the easy "drop in the charger" interface provided by present day desk top battery chargers.